The invention concerns a switching membrane for a pressure control valve as well as a pressure control valve with a switching membrane for controlling or regulating a fluid pressure, in particular for the crankcase ventilation of the internal combustion engine of a motor vehicle.
Pressure control valves are, for example, employed in the venting line between crankcase and intake pipe or air filter of an internal combustion engine. In this context, the goal is to prevent an increase of pressure or vacuum in the containers to be vented past a predetermined value.
In internal combustion engines, blow-by gases are generated in that the combustion gases in the cylinder flow past the cylinder piston into the crankcase. These blow-by gases cause an increase of pressure in the crankcase and may cause leakages and escape of oil. In order to prevent a pressure increase and to discharge the blow-by gases in an environmentally friendly way, they are returned from the crankcase into the air intake manifold of the internal combustion engine. On the other hand, the vacuum should not drop significantly below a specified value because undesired leak air would be sucked into the crankcase due to leaks.
In the pressure control valves that are employed currently, usually a switching membrane of elastomer, often fluorosilicone rubber (FVMQ), is employed. These switching membranes are very flexible as a result of the specific properties of elastomers. As a function of the existing pressure conditions, this switching membrane opens or closes an opening in the pressure control valve. In this context, the switching membrane must react to minimal switching pressures of a magnitude of 100 mbar.
Blow-by gases are comprised of uncombusted fuel portions, motor oil portions, and other pollutants that are produced during combustion. These gases attack many elastomer types so that the material property may become damaged. The components made of these materials become brittle, porous, and cracked. When the switching membranes are damaged, the environmentally detrimental blow-by gases reach directly the environment because the system is no longer seal-tight. The switching membrane made of an elastomer is usually designed as a rolling membrane in order to realize a certain stroke of the membrane. Upon simultaneous contact with blow-by gases, the material is additionally mechanically loaded by the rolling movement within the rolling area and can become damaged in this way.
DE 26 29 621 A1 discloses a membrane valve with a membrane which is clamped at its edge between the valve housing and the housing cover and which, by means of a pressure member, is to be brought into seal-tight contact at a seat surface provided in the valve housing, wherein the membrane is comprised of a thinner layer of minimal elasticity, e.g., of PTFE, facing the housing interior and resistant to aggressive flow media, and a further thicker layer of rubbery-elastic material. Such membrane valves are used primarily where a high chemical resistance of the materials that are coming into contact with the flow medium is required. Since the rubbery-elastic materials do not fulfill these requirements and the chemically resistant materials such as PTFE however do not have the required elasticity for a proper function, membranes that are comprised of two layers are employed. By means of the thick rubber-like layer, the contact pressure that is exerted by the pressure member is transmitted as uniformly as possible onto the sealing surface of the membrane interacting with the seat surface in the valve housing. In this context, for closing the two layer membrane, relatively great pressures of several bars are applied to the membrane by means of a pressure spindle connected to a handwheel in order to ensure the required sealing function by means of the stiff PTFE layer despite the non-existing spring travel of the PTFE layer.